A detector circuit is a circuit which detects the power of a high-frequency signal (RF signal) or other alternating current (AC) signal, and is for example used as a circuit to detect the output power of a power amplifier provided in a portable communication terminal or similar. A power detector circuit performs envelope detection of a high-frequency signal using for example a diode. Such a circuit applies the high-frequency signal to the diode, rectifies the voltage exceeding the diode threshold voltage and converts this into a current, and converts the rectified AC current signal into a direct current (DC) voltage signal using a resistor and a capacitor.
However, the threshold voltage of a normal diode varies due to the influence of temperature, so that use of a simple diode alone for power detection would result in fluctuation of the output level of the power detector circuit.
Japanese Patent Application Laid-open No. 2005-142955 discusses a power detector which eliminates the influence of fluctuation of the diode threshold voltage. In this power detector, an input high-frequency signal is distributed to the anodes of two diodes, a bias current from a bias circuit is supplied to each of the diodes, a DC voltage is generated across a load resistor provided in parallel with the diodes, and the voltage which changes according to the high-frequency signal applied to the diodes is input to a differential amplifier via an inductor. The differential amplifier outputs the difference voltage of two voltages which change according to the high-frequency signal applied to each of the diodes.
The detector circuit discussed in Japanese Patent Application Laid-open No. 2005-142955 computes the difference voltage of two voltages resulting from distribution of the high-frequency signal according to a power distribution ratio determined in advance, and so the output voltage of the differential amplifier is detected as the power level of the high-frequency signal, and even if the threshold voltages of the diodes vary due to temperature changes, the variation in the threshold voltages of the diodes are cancelled by the difference, so that the influence of variation of the threshold voltages is compensated.
However, the above-described power detector uses a differential amplifier. A differential amplifier has a voltage offset due to manufacturing variation, and so there is a limit to the precision of detection of a difference voltage. Further, the output common mode and gain which are characteristics of the differential amplifier tend to fluctuate with changes in the power supply voltage, so the precision of the detection tends to lower by power supply fluctuations.